Internal vs. External SPDs, a
Comparison
The purpose of this document is to compare the
advantages and disadvantages of internal SPD versus
external SPDs. Once the key factors are presented in
this paper an end user can make informed decisions
whether their surge protection devices (SPDs) should
be mounted inside the service equipment or attached
to service equipment via a hardwired external surge
panel.
The National Electrical Code (NEC) definition of
Service Equipment is: “The necessary equipment,
usually consisting of a circuit breaker(s) or switch(es)
and fuse(s), and their accessories, connected to the
load end of service conductors to a building or other
structure, or an otherwise designated area, and
intended to constitute the main control and cutoff of
the supply.”1 Service equipment may consist of
circuit breakers or fused switches provided to
disconnect all ungrounded conductors in a building or
other structure from the service entrance conductors.
INTERNAL PANEL ADVANTAGES
One advantage of an internal SPD is the convenience
of purchasing a single unit to accomplish two
purposes one to provide overcurrent protection and
the second to provide surge protection. The end user
saves cost when creating the purchase order and the
service equipment installation costs are reduced since
the SPD is part of the service equipment. A second
benefit of internal mounted surge protection is that
“minimal wire length is possible because the SPD is
typically connected directly to the service entrance
conductors.”1 The result is lower let-through voltage
or clamping. A third benefit is that the SPD is
connected directly to the energized, bus bypassing
the circuit breaker, which again helps to lower the
clamping voltage. Note: In the IEEE C62.72 it’s
stated that “SPD’s with lower clamping voltage
rating cause frequent interventions of the protective
devices and accelerate their aging. The best approach
or product is not necessarily the one claiming to
suppress surges to the lowest levels”.
INTERNAL PANEL CONCERNS
In Article 240 “Overcurrent Protection” Section 1 (2)
it is stated that “Mounting on open-type
switchboards, panelboards, or control boards that are
in rooms or enclosures free from dampness and easily
ignitable material and are accessible only to qualified
personnel. Metal Oxide Varistors (MOV’s) are
common components incorporated into SPD products
and they fabricated using combustible materials. It’s
not good practice to place a combustible material
within electrical service equipment where a fault
current condition or large surge current could cause
internal damage to the circuit breakers inside the
panel. During a lightning strike, a significant amount
of energy can cause the MOV’s within an SPD to
turn on and conduct current. If the SPD is not sized
appropriately to accommodate the energy contained
in a lightning strike, or a fault current condition could
cause a combustible event. The concern to keep
overcurrent protection away from combustible
materials is mentioned throughout the NEC code.
Some additional references are Article 230.11,
Article 408.7, Article 490.21 and in the Handbook
Article 230-70. Figure 1 below shows an internal
surge panel damaged due to a temporary overvoltage.
Figure 1
SPDs require periodic maintenance. If an SPD is
mounted internal to the service equipment, how will
maintenance be performed? Service equipment is
designed to guard against access to the service
conductors and other energized parts. By definition,
servicing an internal SPD causes forced access into
energized parts within the service equipment. There
are liability concerns related to service equipment
access. Facilities’ personnel are not always
experienced with service equipment maintenance and
its associated safety-related issues. Given this limited
expertise, even electricians who recognize the
hazards posed by electrical service equipment may
not have the equipment i.e. arc flash suit, to maintain
and repair equipment when exposure to energized
service entrance conductors is a factor.
Maintenance of external SPD panels can be
accomplished by turning off the circuit breaker
disconnect and performing the necessary cleaning or
repair on a safe de-energized surge panel. Figure 2
shows an example of an SPD installation featuring an
external panel SPD.
3.
4.
protective device will produce smoke. An
externally mounted SPD reduces the risk of arc
flash and facility downtime.
Visual Status Indicators: A benefit of external
SPDs is that they can provide visual surge
protection status.
Isolates the Damage, Protecting your
Electrical Panel: If combustion occurs due to an
extreme surge event within an externally
mounted SPD, the resulting damage is contained
within the enclosure. This protects the electrical
panel components from potential damage and
reduces the risk of facility downtime.
SPDs designed to protect down-line electronics from
power surges. An external SPD requires separate
dedicated circuit breakers internal to the service
equipment. In an external SPD installation, the downline load doesn’t require the SPD to complete the
network. In the presence of a fault current condition
or MOV failure, the dedicated disconnect will open
and the service equipment will not be damaged.
Figure 2
Another disadvantage of internal SPD's is the limited
space that is available within a service equipment.
There’s limited room within service equipment to
house an SPD and the essential circuit breakers.
When an SPD is installed within service equipment,
the NEC “readily accessible” term for the
disconnecting means could become compromised.
Service entrance cabinets and panel boards are
designed to house overcurrent protective devices and
their energized input and output conductors. When
the same cabinet is used to house an SPD with its
input conductors, the available space will be limited.
EXTERNAL PANEL ADVANTAGES
1.
2.
Easy to Maintain: An externally mounted Surge
Protective Device can be connected to the panel
via a circuit breaker. In the event that the surge
device requires maintenance or replacement, the
breaker is turned off with no impact to facility
power.
Limited Risk of Conductive Residue: In
certain instances, an MOV failure within a surge
A dedicated circuit breaker mounted prior to an
external SPD provides advantages. Circuit breakers
are safe and they contain designed-in isolation
characteristics inherently to prevent exposure to live
parts. Circuit breakers are also designed to open
during an over-current condition and they are
designed to open in the presence of a fault current
condition. Another benefit of circuit breakers are that
they serve as a disconnect for the down-line SPD.
Circuit breakers can be reset, and they have current
limiting characteristics that are beneficial to the
functionality of a well-designed and implemented
SPD product.
IEEE
The IEEE develops standards, guides and
recommended practices for SPDs. Recently the IEEE
has publish two documents below that address safety
concerns related to internal SPDs. The specific
language and references are noted below:
When MOVs reach an end-of-life condition, they
lose their ability to block normal system voltage and
begin to conduct current continuously. The
continuous current condition creates heat. The MOV
initiates a conductive condition identified as thermal
runaway that inevitably results in the destruction of
the MOV. The resulting destruction of the MOV
might expel hot metal fragments, conductive ionized
gases, and dense conductive smoke and soot. In
addition to immediate hazards, the introduction of
such materials into the interior of electrical
distribution equipment can damage or compromise an
insulation system and result in a cascading effect and
serious equipment damage2.
IEEE PC62.72™ [B2], when an SPD is located inside
switchboards or panelboards, there is a concern that
failure of the SPD can cause collateral damage to the
switchboard or panelboard, including compromising
the insulation system with subsequent L-L and L-G
faults3.
CONCLUSION:
After reviewing all the characteristics of the two
approaches to applying surge protective devices it
becomes apparent that an external SPD offers
significant advantages with regard
to safety,
maintenance and performance over the internal SPD.
REFERENCES:
1.
National Electrical Code, 2008 Edition. National Fire
Protection Association Inc., Quincy, Ma 02269-9101
2.
2007 IEEE Guide for the application of
surge protective devices for low voltage
(1000V or less) AC Power Circuits; the
C62.72 document states in Paragraph 14.1:
(page 30)
IEEE-1100 Emerald Book, 8.4.2.5 Surge
Protective Device Considerations: (Page
269)
3.
Kenneth Brown is the Director of Engineering for
C&I San Diego at Leviton Manufacturing Co., Inc.
Ken is currently Chair of the IEEE Surge Protective
Devices Committee, and a member of the UL
Standards Technical Panel. Ken received an
undergraduate in Electrical Engineering from the
Ohio Institute of Technology and an MBA from West
Coast University.